Abstract
Sinorhizobium (Ensifer) fredii (S. fredii) is a rhizobial species exhibiting a remarkably broad nodulation host-range. Thus, S. fredii is able to effectively nodulate dozens of different legumes, including plants forming determinate nodules, such as the important crops soybean and cowpea, and plants forming indeterminate nodules, such as Glycyrrhiza uralensis and pigeon-pea. This capacity of adaptation to different symbioses makes the study of the molecular signals produced by S. fredii strains of increasing interest since it allows the analysis of their symbiotic role in different types of nodule. In this review, we analyze in depth different S. fredii molecules that act as signals in symbiosis, including nodulation factors, different surface polysaccharides (exopolysaccharides, lipopolysaccharides, cyclic glucans, and K-antigen capsular polysaccharides), and effectors delivered to the interior of the host cells through a symbiotic type 3 secretion system.
Highlights
Rhizobia are soil bacteria able to establish a symbiotic association with legumes in which a complex interchange of molecular signals is necessary for a successful infection
We analyze in depth different S. fredii molecules that act as signals in symbiosis, including nodulation factors, different surface polysaccharides, and effectors delivered to the interior of the host cells through a symbiotic type 3 secretion system
An HH103 nodD1 mutant, unable to produce Nod factors (NF), is able to slightly nodulate soybean when it overexpresses ttsI [32]. These results suggest that there is an alternative way to nodulate soybean based on the secretion of effectors through the type 3 secretion system (T3SS)
Summary
Rhizobia are soil bacteria able to establish a symbiotic association with legumes in which a complex interchange of molecular signals is necessary for a successful infection. B. japonicum USDA110 produces LCO that are relatively similar to those of soybean-nodulating S. fredii strains, with the exception than they can carry acetylations in the non-reducing end. The absence of O-methyl-fucose decorations in the HH103 NF by inactivation of the noeL gene does not have a drastic effect on the symbiotic interaction with soybean it reduces the competitiveness to nodulate this legume [15]. In 2007, Giraud and coworkers [30] reported that some photosynthetic bradyrhizobia lacking the nodABC genes required for NF synthesis were able to nodulate some Aeschynomene species, demonstrating for the first time that a NF-independent symbiotic interaction was possible between a legume and a rhizobial strain. These results suggest that there is an alternative way to nodulate soybean based on the secretion of effectors through the T3SS
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